1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
A typical electrophotographic full-color image forming apparatus includes four image forming units that respectively form yellow (Y), magenta (M), cyan (C), and black (BK) toner images. Each of the toner images formed on a surface of an image carrier included in each of the image forming units is primary-transferred onto a surface of an intermediate transfer belt sequentially in a superimposed manner thereby obtaining a full-color toner image. Subsequently, the full-color toner image on the surface of the intermediate transfer belt is secondary-transferred onto a recording medium by a secondary transfer unit such as a transfer roller. In some image forming apparatuses, see, for example, Japanese Patent Application Laid-open No. 2005-266269, an intermediate transfer belt and each of image carriers are made to come in contact with each other only when performing the primary-transfer, and are separated from each other when not performing the primary-transfer for various reasons. Such a configuration makes it possible to improve the durability of the intermediate transfer belt.
An intermediate transfer belt is typically supported by supporting rollers. If an image forming apparatus is not in use for a long time, imprinted marks of the supporting rollers are left on portions of the intermediate transfer belt where the intermediate transfer belt is supported by the supporting rollers. A toner image will be distorted if it is formed on an imprinted mark. FIG. 6 is a schematic diagram of an example of an intermediate transfer belt 115 with two imprinted marks 117. The imprinted marks 117 are generally a few tens of micrometers to a few hundreds of micrometers high depending on curvatures of supporting rollers 120, 121, and the tension on the intermediate transfer belt 115. In some image forming apparatuses, the intermediate transfer belt is supported by three supporting rollers. In other word, the intermediate transfer belt is formed into a triangular shape in a side view. However, an image forming apparatus in which an intermediate transfer belt is supported by three supporting rollers is costlier and bigger than the one in which an intermediate transfer belt is supported by two supporting rollers.